In a spectrophotometer, such as an ultraviolet-visible spectrophotometer or atomic absorption spectrophotometer, a wavelength-dispersion spectrometer (monochromator) is used to produce monochromatic light having a predetermined wavelength. A spectrometer having a common configuration includes a dispersing element, such as a diffraction grating or prism, and a rotary drive mechanism for changing the angle of the dispersing element with respect to the incident light. By appropriately rotating the dispersing element via the rotary drive mechanism, monochromatic light having a desired wavelength can be extracted through an exit slit fixed at a predetermined position. The wavelength-resolving power for this monochromatic light depends on the width of the slit and the accuracy of the angle of the dispersing element. Achieving a high level of wavelength-resolving power requires a rotary drive mechanism capable of accurately rotating the dispersing element by minute steps (Patent Document 1).
Examples of the rotary drive mechanism include a sine-bar drive, cum drive, direct-axis motor drive, and so on. All of these mechanisms use a rotary motion of a motor. In the case of dispersion systems, stepper motors are commonly used because: 1) dispersion systems do not require an extremely high-speed rotation, 2) stepper motors allow easy positioning of the rotation shaft, and for other reasons.